(a) Field of the Invention
The present invention relates to an organic light emitting diode (“OLED”) display and a method for manufacturing the same.
(b) Description of the Related Art
Recently, monitors and television sets have been required to be thinner and lighter. In order to satisfy such requirements, liquid crystal displays (“LCD”) have replaced cathode ray tube (“CRT”) displays.
However, an LCD requires an additional backlight because the LCD is a passive light emitting device. Also, the LCD has problems in terms of response speed and viewing angle.
In order to overcome the problems of the LCD, an organic light emitting diode (“OLED”) display has being receiving attention.
The OLED display includes a light emitting layer interposed between two electrodes, one for injecting electrons and the other for injecting holes to the light emitting layer. The injected electrons and holes are coupled at the light emitting layer, and excitons are thereby formed. The excitons emit light while discharging energy.
The OLED display is itself a light emitting device which does not require an additional light source. Therefore, the OLED display has an advantage of low power consumption. Also, the response speed, the viewing angle, and the contrast ratio of the OLED display are superior.
The OLED display may be classified as a passive matrix OLED display or an active matrix OLED display according to the type of driving mechanism used therein.
The active matrix OLED display includes a switching thin film transistor connected to a signal line which controls a data voltage, and a driving thin film transistor which flows a current into a organic light emitting device by supplying the data voltage received from the switching thin film transistor as a gate voltage.
However, the switching thin film transistor and the driving thin film transistor require different characteristics to allow the OLED display to have optimal performance.
The switching thin film transistor requires a high on/off current ratio (Ion/Ioff), while the driving thin film transistor requires high charge carrier mobility and stability for flowing sufficient and steady current to the organic light emitting device.
If the off-current increases at the switching thin film transistor, the data voltage transferred to the driving thin film transistor is reduced. As a result, cross-talk may arise. Cross-talk is the name for the undesirable effect where information (in the form of voltages) supplied to one row of pixels in the display can affect the image on other pixel rows. If the driving thin film transistor has lower carrier mobility and stability, the amount of current flowing into the organic light emitting device is reduced. As a result, the amount of light emitted from the organic light emitting device is reduced.
However, if the switching thin film transistor and the driving thin film transistor are independently manufactured to satisfy the different required characteristics of each transistor, the number of manufacturing processes and the number of masks would significantly increase. Further, the OLED display must use an additional polarizer to avoid significant degradation of the contrast ratio thereof due to a large amount of reflected external light.